13C Chemical Shift in Natural Gas Hydrates from First-Principles Solid-State NMR Calculations

Abstract

Natural gas hydrates (NGHs) are of interest both as a prospective energy resource and for possible technological applications. 13C NMR technology is a powerful tool to characterize NGHs, and in this work, the trends and origins of 13C NMR chemical shifts of hydrocarbon molecules in NGHs from quantum-chemical first-principles calculations on solid state phases are presented. The chemical shift is found to decrease as the size of the water cavities increases for single occupancy NGHs, and to increase as the amount of CH4 increases for the multioccupancy cases. In most cases, the chemical shift of NGHs monotonically increases as the external pressure increases. Furthermore, the chemical shift can be mainly attributed to the host-guest interaction together with a small contributions from water molecules for tight environments and mainly depends on host-guest interaction for loose environments. The theoretical results provide useful information for identification of the types of clathrate phases and guest molecules included in NGH samples taken from natural sites.